Introduction: Disaccommodation is known to have a shorter time constant and a higher peak velocity than accommodation. Differences in the dynamics suggest dissimilarities in the control strategies for accommodation and disaccommodation. Examining the first (velocity) and second (acceleration) order characteristics of both the systems would provide a better picture of this dissimilarity. Previously, we investigated the first and second order dynamics of accommodation. Here, we present a detailed description of the first and second order dynamics of disaccommodation.

Methods: Disaccommodation responses to a Maltese cross were measured using the SRI Dynamic Infrared Optometer. Five subjects (mean age: 26.2+/− 3.96 yrs) took part in the experiment. Disaccommodative steps ranging from 1–4D were presented monocularly (left eye) for a period of 4secs from a constant starting position of 6D. Position traces were differentiated using a 25msec-differentiating window to obtain velocity and acceleration traces.

Results: Time constants increased with response magnitude (mean slope: 61.15msec/D). Peak velocity was independent of response magnitude. Mean slopes of peak velocity versus response magnitude (slope:−0.011sec-1; P-value: 0.6) and time to peak velocity versus response magnitude (slope:−1.96msec/D; P-value: 0.5) were not significantly different from zero. Upon reaching peak velocity, the responses continued at the peak velocity for duration proportional to the response magnitude (mean slope: 32.66msec/D). The peak acceleration magnitude, time to peak acceleration and the total duration of acceleration were independent of response magnitude.

Conclusions: Peak velocity of disaccommodation is independent of the response magnitude while duration of peak velocity increases proportional to the response magnitude. Larger responses are achieved by driving the system with a force profile that is independent of the response magnitude. The independence between the second order dynamics and response magnitude suggests the existence of an open-loop portion in disaccommodation control system, similar to that proposed by us for accommodation control system.